It is known that the impact strength of various vinylic thermoplastic polymers can be improved by incorporating therein organopolysiloxanes containing alkenyl substituents. This invention concerns a method of further improving the impact strength of vinylic polymers by employing mercaptosiloxanes in lieu of the vinyl-containing siloxanes.
U.S. Pat. No. 3,898,300 shows improving the impact strength of various styrenic copolymers by polymerizing the monomers in an emulsion of a vinylsiloxane-dimethylsiloxane copolymer. Impact strengths of from 0.4 to 7 foot pounds notched Izod were obtained depending upon the amount of siloxane relative to the vinylic polymer and to the amount of vinyl siloxane in the siloxane copolymer. The patent shows that the optimum of about 7 foot pounds is obtained at 20 percent siloxane and the impact strength decreases with additional siloxane to a value of 2.1 foot pounds at 49 percent siloxane (Example 3, C and D). The patent further teaches that optionally RSiO.sub.3/2 and SiO.sub.2 units can be present in the siloxane copolymer in amounts up to 12 and 5 mol percent respectively (column 4, line 55). The patent further teaches in column 5, line 1, that other monovalent radicals may be present and that "some monovalent radicals may even be helpful to polymerization, such as mercaptoalkyl radicals." These can be present in amount of "1 to 2 percent or less." However, there is no suggestion in the patent that mercaptoalkylsiloxane could be used in the place of vinyl or allyl siloxanes nor is there any suggestion that the use of mercaptoalkylsiloxane would improve the impact strength or any other specific property.
U.S. Pat. No. 3,532,729 teaches modifying mercaptoalkylsiloxanes by grafting thereon various olefins, such grafting being carried out in an emulsion of the siloxane. Since this patent is concerned with modifying siloxanes rather than the modification of thermoplastic vinylic polymers, the amount of siloxane disclosed is 50 percent by weight or more with relation to the polymeric vinylic compounds (Examples 2-6). Also, this patent does not show an appreciation of the fact that the amount of crosslinking siloxane, i.e. R"SiO.sub.3/2 or SiO.sub.2 must be limited in order to obtain improved impact strength.
U.S. Pat. Nos. 3,923,923 and 3,879,491 both relate to improving the impact strength of vinylic polymers, but these patents relate to the bulk polymerization of the vinylic monomers with vinyl-containing siloxanes. Although the former shows that some mercaptoalkylsiloxane can be present, neither of these patents suggests the elimination of the vinyl nor the fact that the inclusion of mercaptoalkylsiloxane would improve the impact strength over that which can be obtained by vinyl.
The copending application of James R. Falender and John C. Saam Ser. No. 679,621 entitled "Toughened Mercaptosiloxane Modified Bulk Polymerized Vinylic Polymers" filed concurrently herewith deals with the preparation of improved impact strength vinylic polymers employing bulk or suspension polymerization methods using mercaptosiloxanes. The distinction between that application and the instant one is the fact that this application relates strictly to the preparation of high impact vinylic polymers employing emulsion polymerization techniques.
It is the object of this invention to prepare improved vinylic polymers particularly with respect to impact strength. The vinylic polymers prepared by the process of this invention show less bubbling and less discoloration at elevated temperatures than those prepared using vinylsiloxanes.